This project seeks to continue studies of humoral immunity in mouse and human breast cancer, emphasizing study of distribution of antiviral and antitumor antibodies in sera of tumor-bearing and tumor-free individuals and identification of the role of viral antigens in the humoral response. Emphasis in human studies is on determination of whether antigens in human breast tumors are related to antigens of the mouse mammary tumor virus, and, if so, whether the detected antigens are of etiological significance.